The present invention relates to a device for reshaping of and reducing noise from optical short pulses used in optical transmission systems, optical switching systems, etc.
Saturable absorption elements of utilizing an optical absorption saturation effect of a semiconductor can be applied to reshaping and noise reduction of optical short pulses. The optical absorption saturation effect of semiconductor is known as a property that, in a case where an optical signal of energy near a band-gap energy thereof is injected the semiconductor, its absorption coefficient decreases non-linearly while a transmittance thereof inceases non-linearly as an intensity if injected light increases. This absorption saturation effect is caused by a band filling effect in that electron-hole pairs produced by absorption of light raises a state occupation probability in a band to shift an absorption end to a higher energy side. Accordingly, the saturable absorption element absorbs weak noise when the intensity of the input optical pulse is lower than a threshold value, but it passes therethrough optical pulses of an intensity above the threshold value. Because of such an operation, the saturable absorption element can be used to reshape optical short pulses or reduce therefrom noise.
In a time chart of a transmittance of an optical short pulse in a semiconductor saturable absorption element, a sufficiently fast response on the order of picosecond can be seen at a rise-up instant of the optical short pulse when an absorption coefficient decreases in response to absorption of an incident light. However, at a fall-down time instant of the optical short pulse when an intensity of the injected pulse decreases, a recovery time necessary to a normal absorption coefficient is a time of about a carrier (electron or hole) life time of the order of nanosecond. This is schematically shown in FIGS. 6(a) and 6(b). FIG. 6(a) shows a time chart of an intensity of an optical pulse and FIG. 6(b) a time chart of an absorption coefficient of a saturable absorption element disclosed in a prior application filed by the inventor of this application. For the reason given above, it is impossible with the saturable absorption element to completely eliminate noise from or reshape optical pulses of pulse widths shorter than the carrier life time. Conventional approaches for shortening the carrier life time in a saturable absorption element are, for example, a method for introducing impurities, defects or the like into the absorption layer, a method for extracting carriers from a saturable absorption element by applying a reverse bias to the p-n junction thereof and a method for extracting carriers produced in the absorption region through utilization of the tunnel effect. Yet no semiconductor saturable absorption element has been implemented which is capable of sufficiently responding to optical short pulses of picosecond order.
As mentioned above, conventional devices using such a saturable absorption element have such a drawback that a time necessary to recover a normal absorption coefficient is limited by a carrier life time. Hence, it is impossible with such a saturable absorption element to completely eliminate noise from or reshape optical short pulses of pulse widths shorter than the carrier life time.